Data communications systems that utilize both wired and wireless data communications standards often transmit data in packetized messages. These packetized messages often include a header portion to contain communications protocol related information. Some protocols incorporate header compression (HC) processing to reduce bandwidth utilization, which is particularly useful with wireless links. For example, a standard implementing a Robust Header Compression (RoHC) protocol is designed to efficiency minimize the size of RTP, UDP and IP headers by compressing dynamic fields of these headers using a window based encoding mechanism. In a wireless access network, RoHC can be used to compress various higher layer protocol headers over the radio link. To enable compression of various protocol headers over the radio link, RoHC is implemented on subscriber equipment (e.g. mobile, CPE, etc) and on a node in an access network. Equipment options for implementing the RoHC processing in access network can either be in the base station or in an external, off-the-shelf, radio access router such as PDSN (Packet Data Serving Node), GGSN (Gatway GPRS Support Node), RNC (Radio Network Controller), ASN-GW (ASN Gateway), and the like. RoHC requires the compressor and the de-compressor to establish and maintain context information that is used for compression and de-compression purposes. The compressor and the de-compressor are required to maintain synchronized context to ensure that the de-compressor is able to correctly de-compress the packets compressed by the compressor. Since the context information is crucial for operation of the RoHC protocol, it is essential to use a reliable context update mechanism between compressor and de-compressor.
The RoHC RFC (3095) defines in-band signaling from de-compressor to compressor to enable efficient encoding as well as quick recovery from over the air packet loss and radio link frame corruption containing compressed headers. The in-band signaling is used by the de-compressor to acknowledge the context update signaling received from the compressor. The explicit acknowledgement signaling mechanism makes the header compression protocol self contained and independent of the underlying link layer but it also introduces additional radio link overhead making it less spectral efficient.
The RoHC protocol uses W-LSB encoding to compress various changing filed of RTP/UDP/IP headers. The W-LSB encoding requires compressor to maintain window of reference points that it uses to compress various changing fields of RTP/UDP/IP header. Depending upon the mode of operation, window size is either manually configured or adjusted based upon explicit feedback from the de-compressor to the compressor. The accuracy of W-LSB de-compression depends upon the reference points being used by compressor being synchronized with those used by the de-compressor for de-compression purpose. In the Robust (R)-mode of operation, the RoHC compressor adjusts its window size as soon as it receives feedback from the de-compressor that a particular context update packet containing a given reference point is received by the de-compressor. In case of either U or O mode, the RoHC compressor manually configures the window size and sends the reference update several times to ensure that de-compressor receives the reference points before it discards the reference point from its compressor window.
Although both of these approaches perform their required functionality, they consume communications bandwidth by requiring transmission of these additional synchronization messages. For example, In the R mode of operation, additional messages are sent from the de-compressor to the compressor to update the window size. In case of U or O modes, the W-LSB window size is chosen in such a way that compressor and de-compressor do not get into situation where their reference points are out of sync. This selection of a larger window size reduces the bandwidth efficiency of W-LSB encoding and thereby reduces some of the spectral efficiency gain that is obtained from using the Robust Header Compression protocol.
Therefore a need exists to overcome the problems with the prior art as discussed above.